Lice are an external parasite found on the hair of mammalian hosts, including horses. A lice infestation is also referred to as pediculosis. There are many species of these small, wingless insects, and horses are usually infested with either the “sucking” or “biting” type.

Lice are typically transmitted by direct contact with an infected animal. Since lice can live in the environment without a host for a few weeks, they can also be transmitted indirectly by sharing space or equipment with infected horses.

Symptoms include itching, a patchy coat, inflamed skin, and in severe cases, loss of body condition. Diagnosis is based on the presence of lice and nits (eggs) and may be confirmed with skin scraping or grooming.

Anti-parasitic topical medication is the primary treatment for lice infestations in horses. In severe cases, secondary skin infections may also require antibiotics and wound care.

Lice (Pediculosis) in Horses

Lice are flat, wingless, parasitic insects that live and feed on the body of mammalian hosts, such as horses. [1] Pediciulosis is the medical term for a lice infestation in any animal. [1]

Horses are most often infected during the winter and early spring. Horses with thick coats, young foals, senior horses, pregnant mares, and those who are ill or weak are are at particular risk of pediculosis.

There are over 4000 species of parasitic lice worldwide. Two types of lice can infest horses: sucking lice and biting lice. [1][2][3]

Sucking Lice

The sucking louse species is also known as Haematopinus asini. [3] It feeds on the blood, fluids, and tissues of the horse. [2]

The sucking louse is between 3 and 5 millimeters long. [2][4] It moves slowly and can sometimes be observed with the sucking parts of its mouth embedded in the skin. [2]

Its head and thorax are grey-blue or yellow-brown and it has a wide, pale yellow abdomen ringed with dots. [2][3] The thorax of the sucking louse is much larger than its head and it has distinct mouth parts that are adapted for biting. [2] Its legs are thick and bulbous. [2]

Sucking lice are most likely to be found on the mane, tail, and fetlocks of the horse. [3]

Biting Lice

The biting louse is called Werneckiella equi (previously referred to as Damalina equi and Bovicola equi). [1][2] This species is also known as a chewing louse. [2] W. equi feeds on the skin flakes and hair of the horse, only occasionally taking a blood meal. [2]

The biting louse is between 1 and 2 millimeters long and is more active than other species of lice. [4] Heightened activity makes it easy to spot this species as it moves across the horse’s skin, especially in cases of widespread infestation. [1][4]

In biting lice, the head and thorax are brown with a pale yellow, striped abdomen. [2] The head is square or rounded and broader than the thorax. [2][3] The legs of biting lice are fine and tapered. [2]

Chewing lice can be differentiated from sucking lice by the shape of their body: the sucking lice’s head is narrower than its thorax, whereas the biting louse has a narrow thorax and a larger head. [2]

Biting lice are most likely to be found on the sides of the neck, back, and the abdomen of the horse. [2][3] In cases of heavy infestation they can be found everywhere on the horse’s body. [2]

Parasitic Life Cycle

The life cycle for both species of lice is the same. [2] The louse starts as an nit (egg), develops into a nymph (larva), and then becomes an adult louse. [2]

All stages of the life cycle occur on or just under the skin of the horse. [2] The full cycle takes between 20 and 40 days. [2][3][4]

The nits are small, oval, and translucent. [2] The adult female lays the eggs onto hair follicles near the skin of the horse. [2] As the eggs are laid, they are attached to the individual hairs with a sticky, cement-like substance secreted by the female. [2]

Each female lays one egg per day for the 30 to 35 days of adult life. [2] Eggs take between 5 and 20 days to hatch into nymphs. [2]

The nymphs have a body composition similar to the adult with a head, thorax, and abdomen but they are smaller. [2] It takes a nymph between 14 and 28 days to mature into a louse. [2] The nymphs of sucking lice feed on blood immediately. [2]

Adult lice are wingless and flat which allows them to hide in the horse’s coat. [2] Like the nymphs, they have a head, thorax, and abdomen. [2]

Transmission

Lice are transmitted through direct contact with an infected host when an adult louse crawls from one horse to another. [1][2]

Lice can live for a few weeks off the host. [3] Therefore, transmission can also occur by sharing infected grooming tools, tack, and other equipment or by rubbing against a shared fence post. [2][3][6]

Sharing stall space or using stall space after an infected horse are also possible routes of transmission. [3]

Risk Factors

Lice are ubiquitous, with species found in every part of the world, and are active year-round. [2][3] Lice are more commonly found on horses during the early spring and winter months. [1][2]

Contributing factors to seasonal fluctuations in lice infestation rates may include: [1][2][4]

  • Indoor housing: horses usually have closer contact when kept indoors during cold and rainy weather
  • Increased humidity: most species of insect prolifierate during humid seasons
  • Seasonal changes in coat condition: horses’ coats are longer and thicker during the colder months, providing a more habitable environment for lice to live

General risk factors increasing susceptibility to lice infestation in horses include: [1][2][3][4]